Cr, Ni, Fe, Mo, and C etc. are added to a Co-based alloy used as a heat-resistant material, an wear resistance material, a biomaterial, a medical instrument, or an industrial tool for the purpose of the improvement in corrosion resistance and oxidation resistance, the stabilization of α-phase, and the material strengthening. Further, the Co-based alloy is strengthened by various methods for obtaining required strength such as solid solution strengthening, precipitation strengthening, and work hardening.
The conventional strengthening methods or techniques for improving the quality of materials are based on a metallic structure in which an α-single phase or a second phase is continuously precipitated in the α-phase (Patent documents 1 and 2). However, there is a demand for application directed to thinner wire and miniaturization in addition to severe using environment and much higher strength than the Co alloy strengthened by conventional methods has been required.
The strengthening method by the lamellar structure is also used for other alloy systems and a typical example thereof is a pearlite transformation which is observed in ferrous materials. When the lamellar structure of ferrite and cementite is formed by pearlite transformation, it is highly strengthened so as to satisfy the demand characteristics as piano wires.
As a method for strengthening the quality of materials using the lamellar structure, Cu—Mn—Al—Ni alloy disclosed in Patent document 3 is introduced by the present inventors and further Co—Al binary alloy having the lamellar structure is also reported in Nonpatent document 4.    Patent document 1: JP 7(1995)-179967 A    Patent document 2: JP 10(1998)-140279 A    Patent document 3: JP 5(1993)-25568 A    Nonpatent document 4: P. Zieba, Acta mater. Vol. 46, No. 1 (1998) pp. 369-377
The Co—Al binary alloy having the lamellar structure is a diplophase structure in which a precipitated hard phase is stacked onto a soft α-phase matrix with a very small interlayer spacing in layers and the coexistence of the strength and toughness at high level can be expected. However, the Co—Al binary alloy has an extremely low ductility as compared with usual metallic materials. In the case of cold working at a high workability, cracks from precipitated phase or the boundaries between the α-phase and the precipitated phase are easily generated. As a strategy to overcome the difficulties in working and allow the alloy to be formed into a target shape by cold working such as rolling, drawing, and swaging, it is considered that the working process is divided into multiple stages and strain is removed by intermediate annealing during each process. However, multiple stages of cold working with intermediate annealing lead complication of the production process and higher production cost. Therefore, it cannot be said that it is an effective solution. There is concern that the lamellar structure is disintegrated by intermediate annealing, thereby impairing the characteristics of the lamellar structure in itself.